1. Field of the Invention
The present invention relates generally to a hydrogen absorbing alloy electrode containing hydrogen absorbing alloy powder and a binding agent and a nickel-metal hydride battery using the hydrogen absorbing alloy electrode as its negative electrode, and is particularly characterized in that the binding agent to be used in the hydrogen absorbing alloy electrode is modified so as to attain, in the nickel-metal hydride battery using the hydrogen absorbing alloy electrode as its negative electrode, an increased discharge voltage in a case where the battery is discharged at a high current as well as improved charge/discharge cycle performance.
2. Description of the Related Art
A nickel-metal hydride battery has been conventionally known as one of alkaline storage batteries. Such a nickel-metal hydride battery has been generally employed as its negative electrode a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy.
In fabricating such a hydrogen absorbing alloy electrode, the method conventionally generally utilized is the one so adapted as to prepare a paste by adding a binding agent to hydrogen absorbing alloy powder and then apply the paste to a current collector composed of a punching metal or the like.
However, when the paste comprising the hydrogen absorbing alloy powder containing the binding agent added thereto is applied to the current collector as described above, contact between each hydrogen absorbing alloy powder is insufficient and further, contact between the hydrogen absorbing alloy powder and the current collector is degraded, resulting in increased resistance in a hydrogen absorbing alloy electrode. Accordingly, in a nickel-metal hydride battery employing such a hydrogen absorbing alloy electrode as its negative electrode, a discharge voltage in a case where the battery is discharged at a high current is decreased and charge/discharge cycle performance is degraded.
Therefore, in recent years, Japanese Patent Laid-Open No. Heil0(1998)-241693 has proposed to use, as a binding agent in a hydrogen absorbing alloy electrode, a copolymer of aromatic vinyl units, conjugated diene units, (meta)acrylic ester units, and functional group-containing compound units, in which the content of the (meta)acrylic ester units is in the range of 10 to 40% by weight of the whole copolymer. This binding agent serves to enhance binding between hydrogen absorbing alloy powder and a current collector in the hydrogen absorbing alloy electrode and hence, a nickel-metal hydride battery employing the hydrogen absorbing alloy electrode as its negative electrode is improved in charge/discharge cycle performance and the like.
Unfortunately however, a hydrogen absorbing alloy electrode using as a binding agent the copolymer as disclosed in the above-mentioned gazette suffers insufficient water retentivity. As a result, in a nickel-metal hydride battery employing such a hydrogen absorbing alloy electrode, there still remain problems that a discharge voltage in a case where the battery is discharged at a high current can not be sufficiently increased and that charge/discharge cycle performance is still inferior.
An object of the present invention is to enhance, in a hydrogen absorbing alloy electrode containing hydrogen absorbing alloy powder and a binding agent, the binding between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and a current collector as well as water retentivity of the hydrogen absorbing alloy electrode.
Another object of the present invention is to attain, in a nickel-metal hydride battery employing the above-mentioned hydrogen absorbing alloy electrode as its negative electrode, an increased battery voltage when the battery is discharged at a high current as well as improved charge/discharge cycle performance, and to prevent a rise in internal pressure of the battery when the battery is overcharged.
A hydrogen absorbing alloy electrode according to the present invention is a hydrogen absorbing alloy electrode containing hydrogen absorbing alloy powder and a binding agent, wherein said binding agent is a copolymer of aromatic vinyl and at least one of acrylic ester and methacrylic acid ester and the total content of acrylic ester units and methacrylic acid ester units in the copolymer is in the range of 43 to 90% by weight.
The total content of acrylic ester units and methacrylic acid ester units in the above-mentioned copolymer is set in the range of 43 to 90% by weight because when the total content is less than 43% by weight, flexibility of the copolymer is degraded. If the binding agent comprising the copolymer with such degraded flexibility was used to apply hydrogen absorbing alloy powder to a current collector, it is considered that the hydrogen absorbing alloy powder is liable to fall off from the current collector and hence, binding between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector is degraded, whereby resistance in a hydrogen absorbing alloy electrode is increased and an alkaline electrolyte can not be well dispersed in the hydrogen absorbing alloy electrode. On the other hand, when the total content of acrylic ester units and methacrylic acid ester units is more than 90% by weight, the copolymer is easily dissolved in an alkaline electrolyte. It is considered that when the copolymer is dissolved in the alkaline electrolyte, binding between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector is degraded, whereby resistance in the hydrogen absorbing alloy electrode is increased and oxygen gas generated during overcharge can not be sufficiently absorbed in the hydrogen absorbing alloy powder.
As in the hydrogen absorbing alloy electrode according to the present invention, when the copolymer of aromatic vinyl and at least one of acrylic ester and methacrylic acid ester, in which the total content of acrylic ester units and methacrylic acid ester units is in the range of 43 to 90% by weight of the whole copolymer is used as the binding agent, sufficient contact between hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector can be attained, so that resistance in the hydrogen absorbing alloy electrode is reduced. Further, water retentivity of the hydrogen absorbing alloy electrode is improved by the acrylic ester and/or methacrylic acid ester contained in the copolymer, and oxygen gas generated during overcharge can be sufficiently absorbed in the hydrogen absorbing alloy powder.
In a nickel-metal hydride battery employing the hydrogen absorbing alloy electrode as its negative electrode, a discharge voltage in a case where the battery is discharged at a high current is increased, charge/discharge cycle performance is improved, and internal pressure is prevented from rising when the battery is overcharged.
In the above-mentioned nickel-metal hydride battery, in order to increase the discharge voltage in a case where the battery is discharged at a high current as well as to improve the charge/discharge cycle performance, the total content of acrylic ester units and methacrylic acid ester units in the above-mentioned copolymer is preferably set in the range of 50 to 90% by weight, and more preferably 56 to 70% by weight.
Further, in the above-mentioned nickel-metal hydride battery, in order to prevent a rise in internal pressure during overcharge, the total content of acrylic ester units and methacrylic acid ester units in the above-mentioned copolymer is preferably set in the range of 46 to 70% by weight, and more preferably 46 to 56% by weight.
In obtaining the above-mentioned copolymer, examples of the above-mentioned acrylic ester and methacrylic acid ester include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, hexyl acrylate, hexyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxymethyl acrylate, 2-hydroxymethyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and the like. Among these, methyl acrylate and methyl methacrylate are particularly preferred.
Further, examples of the above-mentioned aromatic vinyl include styrene, methylstyrene, chlorostyrene, divinylbenzene, and the like. Among these, styrene is particularly preferred.
In the hydrogen absorbing alloy electrode according to the present invention, when the amount of the above-mentioned copolymer in the hydrogen absorbing alloy electrode is too small, binding between each hydrogen absorbing alloy powder is degraded, resulting in insufficient contact between each hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the current collector. On the other hand, when the amount of the above-mentioned copolymer is too large, the copolymer presents between each hydrogen absorbing alloy powder in excess, thereby preventing sufficient contact between each hydrogen absorbing alloy powder. In either one of the cases, resistance in the hydrogen absorbing alloy electrode is increased, thereby decreasing a discharge voltage in a case where the nickel-metal hydride battery employing the hydrogen absorbing alloy electrode is discharged at a high current. Therefore, the content of the above-mentioned copolymer in the hydrogen absorbing alloy electrode is preferably set in the range of 0.2 to 1.0% by weight based on the weight of the hydrogen absorbing alloy powder.
The above-mentioned copolymer may contain impurities and the like. It is thus preferable to heat-treat the copolymer so that the impurities and the like contained therein is evaporated to be removed. However, when the temperature at which the copolymer is heat-treated is too high, molecular chains in the copolymer may be separated. Therefore, it is preferable to heat-treat the above-mentioned copolymer at temperatures in the range of 100 to 180xc2x0 C.
When the copolymer is heat-treated to remove the impurities and the like contained therein as described above, the contact between each hydrogen absorbing alloy powder is further enhanced, whereby a discharge voltage in a case where the battery is discharged at a high current is further increased.
As the above-mentioned copolymer, it is preferable to use a copolymer having a glass transition point (Tg) of not less than 10xc2x0 C. At temperatures lower than this glass transition point, the copolymer is in a hard state like glass and hence, hydrogen absorbing alloy powder can be firmly retained in the hydrogen absorbing alloy electrode by the binding agent. Accordingly, even in a case where a nickel-metal hydride battery is discharged under low temperature conditions, the battery can be discharged at a high voltage.
It should be noted here that the type of hydrogen absorbing alloy to be used in the hydrogen absorbing alloy electrode according to the present invention is not particularly limited. Examples of a usable hydrogen absorbing alloy include a misch metal nickel hydrogen absorbing alloy; Zrxe2x80x94Ni type hydrogen absorbing alloy such as ZrNi; Tixe2x80x94Fe type hydrogen absorbing alloy such as TiFe; Zrxe2x80x94Mn type hydrogen absorbing alloy such as ZrMn2; Tixe2x80x94Mn type hydrogen absorbing alloy such as TiMn1.5; and Mgxe2x80x94Ni type hydrogen absorbing alloy such as Mg2Ni.
Such hydrogen absorbing alloys may be fabricated by a conventionally known method, for example, a melt-quenching method, an atomizing method, and a high-frequency melting method.
As the above-mentioned misch metal nickel hydrogen absorbing alloy, for example, the one represented by a constitutional formula of MmNiaCobAlcMnd can be used. In the constitutional formula, Mm denotes a mixture of elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Sc, Y, Pm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Particularly, it is preferable that Mm comprises a mixture mainly containing elements selected from the group consisting of La, Ce, Pr, Nd, and Sm. Further, the above-mentioned a, b, c, and d, which represent an atomic ratio, satisfy the conditions of a greater than 0, b greater than 0, c greater than 0, dxe2x89xa70, and 4.4xe2x89xa6a+b+c+dxe2x89xa65.4, and preferably further satisfies the conditions of 2.8xe2x89xa6axe2x89xa65.2, 0 less than bxe2x89xa61.4, 0 less than cxe2x89xa61.2, and dxe2x89xa61.2. Furthermore, it is preferable that the above-mentioned c and d satisfy the conditions of cxe2x89xa61.0 and dxe2x89xa61.0 in order to increase a battery capacity of a nickel-metal hydride battery.
When the particle diameter of the above-mentioned hydrogen absorbing alloy powder is too small, the surface area of the hydrogen absorbing alloy powder becomes large, whereby the surface of the hydrogen absorbing alloy powder is liable to be oxidized. When the surface of the hydrogen absorbing alloy powder is oxidized, the hydrogen absorbing alloy powder can not achieve sufficient electrical contact with each other, resulting in degraded conductivity of the hydrogen absorbing alloy electrode. On the other hand, when the particle diameter of the above-mentioned hydrogen absorbing alloy powder is too large, the surface area of the hydrogen absorbing alloy powder becomes small, whereby the area involved in the reaction is decreased. Accordingly, in either one of the cases, overpotential in the nickel-metal hydride battery is increased, thereby decreasing a discharge voltage in a case where the battery is discharged at a high current. It is thus preferable to use hydrogen absorbing alloy powder having an average particle diameter in the range of 10 to 70 xcexcm.
Further, in fabricating a hydrogen absorbing alloy electrode by applying a paste prepared by mixing the above-mentioned hydrogen absorbing alloy powder and binding agent to a current collector, a thickener comprising a water soluble high polymer such as polyethylene oxide is preferably added to the above-mentioned paste so as to facilitate uniform application of the paste to the current collector.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.